Natural gas, and other combustible gases, are routinely transported to users from remote locations via high pressure pipelines. At the user location, it is conventional to apply the high pressure gas to a throttle valve that serves to reduce the pressure of the gas to a level compatible with local distribution and/or use.
Throttling of a high pressure gas wastes the energy expended in the process; and it is presently preferable to recover some of this energy by expanding the gas in an expander rather than by throttling the gas. Here, the high pressure gas expands in a rotary machine such as a turbine coupled to a generator, and some of the pressure reduction is converted to electricity.
Because a temperature drop accompanies the pressure drop through the expander, any moisture in the gas is likely to freeze detrimentally affecting the operation of the expander and utilization devices downstream of the expander. It has been suggested, therefore, to preheat the gas before it is applied to the expander, and to this end, it is conventional to burn fuel for this purpose.
Preventing the freezing of water vapor in the gas is obviously very beneficial from a system standpoint; but warming the gas prior to its expansion does not address another serious problem when the gas being expanded is combustible. The rotary speed of the shaft carrying the expander disk is conventionally very high, and in fact, is much higher than the synchronous speed of the generator to which the shaft is coupled. Combined with the pressure of the gas, the shaft speed of the expander makes it very difficult to effect a perfect seal and prevent leakage. Such leakage, in general, is very dangerous because of the potential for explosion. Leakage in the vicinity of an electric generator is so hazardous as to cause consideration of precluding the use of a generator in the vicinity of an expander operating on combustible gas.
One potential solution to this problem is suggested in U.S. Pat. No. 3,909,012 which discloses a compressor for liquefying natural gas. In this patent, the shaft carrying the compressor is provided with seals adjacent the compressor rotor. Pressurized gas flows away from the compressor through each seal and into a central chamber in the seal which is connected to a pump that extracts the gas and delivers it to a burner. Nitrogen and/or other inert gas is pumped into each seal on the other side of the central chamber so that the nitrogen or inert gas flow in one direction toward the compressor and into the chamber, and in the other direction into a collection chamber which is vented.
While this approach may be viable so far as inhibiting explosions, the approach is somewhat complicated and does not address the problems caused when moisture is present in the combustible gas and freezing occurs during expansion. It is therefore an object of the present invention to provide a new and improved method of and apparatus for reducing the pressure of a high pressure combustible gas, which method and apparatus are less complex than those known in the prior art, and prevent freezing of water vapor in the combustible gas during its expansion.